With functional examinations in particular, it is usual in modern magnetic resonance facilities to record the ECG of the patient in parallel with the magnetic resonance examination thereof. In addition to monitoring, gating processes can be controlled by means of these ECG signals for instance.
ECG electrode systems were thus developed especially for magnetic resonance facilities, said ECG electrode systems being easy to operate and robust and also allowing a reliable ECG derivation by reducing interfering influences as a result of movement or magnetic field effects.
The use of three ECG electrodes, which are attached ventrally very close to one another on the thorax, was proposed for instance. As a result of the minimal electrode distance, the sensitivity in terms of gradient or movement-induced effects thus reduces. Fiber optic cables which likewise prevent influences from magnetic fields are used in most cases for ECG signal derivation.
One problem however is and remains the positioning of the in particular three ECG electrodes such that an evaluable ECG signal is obtained, because the very close placement of the three ECG electrodes in respect of one another, as shown, significantly influences the quality of the derived ECG signal. If the electrodes are placed such that the heart axis draws an unfavorable angle in respect of the electrode pairs or if they are positioned on an unsuitable point relative to the heart for instance, the derived signals are frequently too weak or qualitatively unsuited to a stable evaluation of the ECG signal which is used particularly for triggering or gating purposes.
Nowadays it is thus usual for the electrode positions to be improved when a non-evaluable ECG signal is obtained, by repeatedly changing the attachment positions of the electrodes, according to a “trial and error” principle, until a satisfactory ECG signal can be detected.